The human pathogen Vibrio cholerae is widely distributed along many coastlines including those of the United States. Environmental factors play a major role in determining the distribution of toxigenic V. cholerae. For example, the introduction of the seventh pandemic of cholera to Latin America in 1991 has been suggested to have been correlated with marine plankton blooms triggered by a climate change event such as that initiated by El Nino. In a effort to better understand marine vectors and conditions influencing the spread of cholera, we propose to examine the association of a variety of clinical and environmental strains of V. cholerae with marine plankton. Marine mesocosm experiments will be performed in which V. cholerae will be identified by marking cells with a modified version of the green fluorescent protein gene. growth of V. cholerae in the mesocosm will be followed using the frequency of dividing cells technique coupled to an image analysis method. Using selected marine model systems V. cholerae plankton colonization will be further examined, and the possible role of chemotaxis in plankton or phytodetritus association will be assessed. We will also isolate and characterize, or obtain mutants deficient in plankton colonization. The genetic defects will be identified, and the relationship, if any, between colonization of plankton and of mammals will be ascertained. Finally, the effect of biological and physiochemical factors on V. cholerae growth and distribution in mesocosms will be determined and the possibility of cholera toxin phage production and toxin gene exchange among cells in the marine environment will be explored. By understanding how biological, chemical, and physical factors influence the distribution, abundance and virulence of V. cholerae, together with the elucidation of the genetic requirements for its persistence with plankton in coastal ecosystems, it will be possible to better predict when environmental change is likely to present a cholera public health risk.